As one step of processes for manufacturing a semiconductor device, there is a thin film formation step in which a thin film, e.g., a silicon nitride film (Si3N4 film), is formed on a substrate such as a semiconductor wafer. The thin film formation step is performed by supplying a process gas into a process vessel containing a substrate. Although an object of the thin film formation step is to form a thin film on a substrate, in reality, a deposit including a thin film adheres to portions in addition to the substrate, e.g., the inner wall of the process vessel. The thickness (or amount) of a deposit adhering to the process vessel is gradually increased (e.g., in a cumulative manner) each time the thin film formation step is performed. If the thickness of such deposit reaches a certain level, a part of the deposit may be peeled from the inner wall of the process vessel. This may cause the generation of foreign substances (e.g., particles). If such foreign substances are generated within the process vessel and falls on the substrate, it may reduce a product yield rate of the manufacturing process. Therefore, the inside of the process vessel needs to be cleaned by removing any deposit formed thereon if the thickness of such deposit reaches a certain level.
Prevailing in the past were wet cleaning methods in which a member such as a reaction tube making up a process vessel is taken out from a substrate processing apparatus and then soaked in a cleaning tank containing an aqueous HF solution to remove a deposit adhering to the inner wall of the reaction tube. In recent years, dry cleaning methods have been widely used, which eliminate the need to take out a reaction tube or other members. In the dry cleaning methods, no operation is required to detach the reaction tube from the substrate processing apparatus. Further, any damage to the reaction tube or other members as a result of detaching can be prevented, which leads to the reduction of maintenance cost. Moreover, the dry cleaning methods shorten the time required to perform the cleaning operation until the thin film formation step is resumed. This may increase the productivity of the substrate processing apparatus. For example, in one of the dry cleaning methods, a cleaning gas including a fluorine-containing gas such as a nitrogen trifluoride (NF3) gas, a fluorine (F2) gas, or a chlorine trifluoride (ClF3) gas is thermally activated and supplied into a process vessel (see International Publication No. WO2007/116768).
In a dry cleaning method, the etching rate of a deposit may be increased as the temperature within a process vessel is increased when a cleaning gas is supplied into the process vessel. However, the etching rate of quartz used as a material of a member such as a reaction tube arranged within the process vessel is also increased in proportion to the increase in the temperature within the process vessel. This poses a problem in that foreign substances are generated due to the damage to quartz. On the other hand, the damage to quartz may be reduced as the temperature within the process vessel becomes lower. However, this entails another problem of lowering the etching rate of a deposit within the process vessel. Accordingly, there is a need to realize a cleaning operation meeting two conflicting requirements, i.e., reduction of damage to quartz and increase of deposit etching rate.